By virtue of recent advances in the technology of the elastic substratum method (ESM) it is now possible to generate accurate vector maps of the traction stresses exerted against a substratum due to the cytoskeletal activity of a single biological cell. We here propose to exploit th ESM so as to answer several fundamental questions about the biological significance of traction stresses, about the physical and molecular origin of these stresses and about the dynamics and control of the cytoskeleton in fibroblasts. AIM number 1 to test the role of traction in the modalities by which cells sense and respond to mechanical perturbations. This will be accomplished by measuring traction stresses during and following controlled deformations to the substratum. AIM number 2 to analyze the dynamics of traction, motion and shape in pairs of normal and transformed cells as they come into contact. AIM number 3 to invent, analyze and test a variety of quantitative mechanical models of the mechanism and molecular origin of the fibroblast-substratum traction stress. AIM number 4 to test the functional linkage between the transmission to traction forces to the substratum, the presence of various integrin isotypes and the presence of Zxyin containing adhesion sites.